After we have a complete knowledge of the nature of amino acid residues in the hypervariable segments as to whether they are contacting or structural, we shall attempt to predict the three-dimensional folding of these segments. A number of immunoglobulins have known antigenic specificities, and the structures of the antigen molecules will also be incorporated into such predictions. Recently, the tertiary structure of the neurotoxins has been determined by X-ray. As suggested by various spectroscopic measurements, it contains extensive beta-sheets and no alpha-helix. We have thus initiated a study to try to refine our method of selecting (phi, psi) angles in the sense that if a priori a protein is known to contain only beta-sheets, the reference proteins used in the angle selection should probably be of the same type of proteins. Apparently, this is a subtle restriction on the folding of protein backbones, and may be incorporated into our predictive methods. BIBLIOGRAPHIC REFERENCES: E.A. Kabat, T.T. Wu and H. Bilofsky, Proc. Nat. Acad. Sci. USA, 73, 617 (1976). M.E. Johnson and T.T. Wu, J. Theoret. Biol., 60, 183 (1976).